Lake Tahoe is a large freshwater lake in the Sierra Nevada of the United States. Visibility in Lake Tahoe was about 100 feet in the 1970s, but has since declined. The main culprits seem to be dirt, dust and other fine particles. Lake Tahoe’s clarity improved in 2012 for the second year in a row, and its waters were the clearest in 10 years, according to University of California, Davis, scientists who study the lake. Last year’s average annual clarity level was 75.3 feet, or a 6.4-foot improvement from 2011, according to data released today by the UC Davis Tahoe Environmental Research Center and the Tahoe Regional Planning Agency.

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Despite land-use planning and export of treated sewage effluent from the basin, the lake is becoming increasingly eutrophic (having an excessive richness of nutrients), with primary productivity increasing by more than 5% annually, and clarity decreasing at an average rate of 0.25 meters per year. Some have concluded that suspended inorganic sediments and phytoplanktonic algae both contribute significantly to the reduction in clarity, and that suspended particulate matter is the dominant causes of clarity loss. The largest source of fine sediment particles to Lake Tahoe is urban storm water runoff, comprising 72 percent of the total fine sediment particle load.

Until recently, construction on the banks of the Lake had been largely under the control of wealthy real estate developers. Construction activities have resulted in a clouding of the lake's blue waters. Currently, the Tahoe Regional Planning Agency is regulating construction along the shoreline.

The lake’s clarity is measured by the depth at which a 10 inch white disk, called a Secchi disk, remains visible when lowered beneath the water’s surface. The measurements have been taken since 1968, when the Secchi disk could be seen down to an average of 102.4 feet.

The annual clarity level is the average of 22 individual readings taken throughout the year. The highest individual value recorded in 2012 was 107 feet, and the lowest was 57 feet.

Researchers provided measurements for last year’s winter (December—March) and summer (June—September) months. Winter clarity last year continued a long-term pattern of improvement, with the best clarity since 1996. The winter average of 88.3 feet in 2012 was well above the worst point seen in 1997 and a combined 12-foot improvement for the past two years.

At 64.4 feet, summer clarity improved 13 feet from the 2011 value, but researchers say the persistent trend is still one of declining summer clarity.

"The improvement we see in both the summer and winter clarity during 2012 is very encouraging," said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. "The lake will continue to be subjected to a range of disturbances, each of which has the potential to impact clarity. There is now growing belief that managing for clarity is possible."

This year’s value is an improvement of more than 10 feet on that period, but still 22 feet short of the clarity restoration target of 97.4 feet set by federal and state regulators.

Urban storm water runoff has long been a contributor to reduced clarity at the lake. Most of that runoff occurs during the winter and spring, when rain and snow melt carry small, inorganic particles from the land, roads and other developed areas into the lake.

Yet, despite wet winters during 2011 and 2012, clarity improved. Researchers say this could indicate that efforts led by the Tahoe Regional Planning Agency, other management agencies, local jurisdictions and private property owners to reduce urban storm water runoff are having a positive impact. However, the researchers emphasize that they need more data on storm water to make more definitive conclusions.

"We are very excited about the results from 2012, especially within the context of the long-term record for annual and winter clarity," said John Reuter, associate director of the UC Davis Tahoe Environmental Research Center. "It is particularly encouraging to see clarity improve during wet years when the amount of fine sediments and nutrients going into the lake is high."